1. Field of the Invention
The present invention relates to a complementary metal-oxide semiconductor (CMOS) image sensor, and more particularly, a method for fabricating a CMOS image sensor that reconditions and/or protects a surface of a photodiode and improves characteristics of the image sensor.
2. Discussion of the Related Art
Generally, a CMOS image sensor is a device that converts optical images, i.e., incident light, to electrical output signals using an array of pixels, each unit pixel comprising a P-N junction photodiode and accompanying CMOS transistor circuitry.
In view of the large number of pixel units per array, CMOS-image sensors employ a switching mode that sequentially detects electrical output signals for each unit pixel using MOS transistors in a number directly proportional to the number of pixels. An important consideration in a CMOS image sensor is to have high sensitivity to incident light.
CMOS image sensors have several advantages over charge coupled device (CCD) image sensors, which are widely used, such as a simple driving mode, various scanning modes, miniaturization of the product because of or through signal processing circuit integration in a single chip, low fabricating cost owing to compatible CMOS techniques, and low operating power consumption.
CMOS image sensors are classified by the number of transistors utilized in each pixel, including a three-transistor (3T) type, a four-transistor (4T) type, and a five-transistor (5T) type. The 3T type CMOS image sensor includes a photodiode and three transistors. The 4T type CMOS image sensor includes a photodiode and four transistors. The 5T type CMOS image sensor includes a photodiode and five transistors. An equivalent circuit and a layout of a unit pixel of the 4T type CMOS image sensor will be described with reference to FIGS. 1-2.
FIG. 1 is a circuit diagram illustrating a related art 4T type CMOS image sensor. FIGS. 2A-2C are cross-sectional views illustrating part of the fabrication process of the related art CMOS image sensor.
A general related art 4T type CMOS image sensor, as shown in FIG. 1, includes a unit pixel consisting of a photodiode and four MOS transistors. The CMOS image sensor includes a photodiode 100 generating electrical charges responsive to incident light, a transfer transistor 101 transferring the charges collected in the photodiode 100 to a floating diffusion area 102 in response to signal Tx, a reset transistor 103 setting the potential of the floating diffusion area 102 at a desired value and emitting charges to reset the floating diffusion area 102 in response to signal Rx, a drive transistor 104 serving as a source follower buffer amplifier, and a select transistor 105 addressing a switching mode in response to signal Sx. A load transistor 106 is formed outside the unit pixel to read output signals (or set the potential of the output node at a desired value, such as a ground potential) in response to signal RL. 
A related art method for fabricating a CMOS image sensor will now be described with reference to the accompanying drawings.
FIGS. 2A to 2C are sectional views illustrating a related art process of fabricating a CMOS image sensor containing four transistors and a photodiode. The related art CMOS image sensor fabrication illustrations focus on the photodiode area and a transfer transistor adjacent to the photodiode area.
First, as shown in FIG. 2A, a gate electrode 12 is formed on a semiconductor substrate 11, generally with a gate oxide film therebetween. A barrier oxide film 13 is formed on substrate 11 to minimize surface leakage of the semiconductor substrate 11 during various ion implantation processes performed on the substrate 11.
Afterwards, although not shown, a photoresist pattern may be formed to expose portions of the semiconductor substrate 11 to ion implantation to form a lightly doped drain (LDD) area and N type source/drain (NSD) areas. The LDD area and the NSD areas are formed using the photoresist pattern as an ion implantation mask.
Next, as shown in FIG. 2B, a photoresist pattern PR is formed, leaving exposed a portion of the semiconductor substrate 11 having barrier oxide film 13 thereon. Photodiode area PD 14 is formed in the exposed portion of the semiconductor substrate 11 using the pattern PR as an ion implantation mask.
As shown in FIG. 2B, the photodiode area 14 is formed at an edge of the gate electrode 12 of the transfer transistor Tx in such a way that one side of each is aligned with the other. Thereafter, pattern PR is removed.
Subsequently, as shown in FIG. 2C, a spacer 12a is formed at (both) sidewalls of the gate electrode 12 by depositing an insulating film for the spacer and anisotropically etching the entire insulating film. The barrier oxide film 13 is then removed by a wet etching process.
Several identifiable problems exist with the related art fabrication process for CMOS image sensors. As discussed, after the gate electrode 12 and the barrier oxide film 13 are sequentially formed, ion implantation processes are sequentially performed to form the LDD area, the NSD areas and the photodiode area 14. However, the photodiode surface of the unit pixel may be damaged by several processes of forming and removing photoresist and the etching process for removing the oxide film. This could lead to a leakage source.
As described above, dislocation(s) of the silicon lattice in the semiconductor substrate 11 may be caused when surface characteristics of the photodiode PD 14 deteriorate. Moreover, the dislocated portion of the silicon lattice structure may serve as an electron trap that may capture electrons. The electrons captured in the electron trap affect image display through the transfer transistor Tx. As a result, characteristics of the CMOS image sensor may deteriorate.